A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. The strokes are typically denoted as compression and power strokes. In a two-stroke cycle, opposed-piston (“OP2S”) engine two pistons are disposed crown-to-crown in the bore of a cylinder for reciprocating movement in opposing directions along the central axis of the cylinder. The cylinder has longitudinally-spaced inlet and exhaust ports formed in the cylinder sidewall near respective ends of the cylinder. Each of the opposed pistons controls a respective one of the ports, opening the port as it moves toward a bottom dead center (BDC) location during a power stroke (also called an expansion stroke), and closing the port as it moves from BDC toward a top dead center (TDC) location during a compression stroke. One of the ports provides passage for the products of combustion out of the bore, the other serves to admit pressurized air into the bore; these are respectively termed the “exhaust” and “intake” ports (in some descriptions, intake ports are referred to as “air” ports or “scavenge” ports).
OP2S engines typically operate according to the compression-ignition principle. During a compression stroke, turbulent pressurized air (“charge air”) enters the bore of a cylinder through the intake port and is compressed between the end surfaces of the two pistons as they move from BDC toward TDC. Fuel directly injected into the cylinder between the approaching piston end surfaces mixes with the turbulent air. The fuel is ignited by the heat of the compressed air, and combustion follows. Fuel is provided by an engine fuel handling system that includes one or more fuel injectors mounted to the cylinder between the TDC locations of the piston end surfaces.
In a uniflow-scavenged OP2S engine, near the end of a power stroke, charge air entering a cylinder through the intake port displaces exhaust gas flowing out of the cylinder through the exhaust port. Thus gas flows through the cylinder in one direction (“uniflow”)—from intake port to exhaust port. A continuous positive pressure differential must exist from the intake ports to the exhaust ports of the engine in order to maintain the desired unidirectional flow of gas in the cylinders. Further, a high air mass density must be provided to the intake ports because of the short time that they are open; this need is especially acute during engine start, acceleration, and load increases. This requires pumping work.
In an opposed-piston engine, the pumping work is done by an air handling system (also called a “gas exchange” system) which moves fresh air into and transports combustion gases (exhaust) out of the engine. The pumping work may be done by a gas-turbine driven compressor (e.g., a turbocharger), and/or by a mechanically-driven pump, such as a supercharger (also called a “blower”). In some instances, the compressor may be located upstream or downstream of a supercharger in a two-stage pumping configuration. The pumping arrangement (single stage, two-stage, or otherwise) can drive the scavenging process, which is critical to ensuring effective combustion, increasing the engine's indicated thermal efficiency, and extending the lives of engine components such as pistons, rings, and cylinder.
During steady state performance of an OP2S engine, operational parameters change slowly, if at all. Thus, for example, when propelling a vehicle on a highway at a steady speed, the transport of gasses (charge air and exhaust) through, and provision of fuel in, the vehicle's OP2S engine can be maintained at a slowly-changing pace. This translates to stable control with enough time to optimize engine performance in terms of fuel efficiency and emissions. However, vehicle operation frequently subjects the engine to sudden demands for torque, especially in urban driving or during operation in industrial conditions. Such demands may come from acceleration, deceleration, switching accessories (like air conditioning) on or off, pulling a trailer, climbing a hill, and so on. A sudden demand for torque associated with an abrupt change in engine load or engine speed is considered to be a transient event. Such a demand is hereinafter referred to as a “torque request” During a transient event, a demand for increased torque generates a requirement to quickly increase the supply of fuel to the engine in order to raise the level of energy released by combustion. This requires a concurrent provision of additional air in order to burn the additional fuel.
It is desirable to limit the production of emissions during engine operation. Consequently, during a transient event, a limiting factor for OP2S engine response may be defined by how rapidly the air handling system can change the flow of charge air through the engine in support of a torque request while keeping engine emissions under control. During the period of the torque request, a low air/fuel ratio (AFR) value due to the lack of charge air can result in incomplete combustion, leading to particulate matter (PM) emissions, such as soot. On the other hand, reducing the fuel supply to maintain a target AFR can result in poor engine response.
In a uniflow-scavenged OP2S engine, some of the air delivered to a cylinder during a cycle of engine operation (“delivered air”) flows out of the exhaust port during scavenging and thus is not available for combustion. An accurate measure of AFR for use in controlling combustion uses the mass of charge air retained (“trapped”) in the cylinder when the last port of the cylinder is closed. Depending on engine design either the exhaust port or the intake port may be the last to close; in many instances, the intake port is the last to close. It is further the case that, in addition to the trapped charge air, a measurable mass of residual exhaust gas may sometimes be trapped in the cylinder by closure of the exhaust port and/or by recirculation into the cylinder with the charge air.
Provision of fuel and air in the engine is governed by an engine control mechanization that senses various engine operating parameters and regulates the flow of gasses (air and exhaust) through the engine and the injection of fuel into the engine. It is particularly desirable that the engine control mechanization be able to recognize transient events of an OP2S engine so as to rapidly configure the air handling system for increasing the amount of delivered and/or retained charge air provided to the cylinders in response to torque requests.
The gas pressure differential across the engine that is necessary to sustain the unidirectional flow of charge air and exhaust is generated and sustained by air handling elements of the air handling system, which may include a supercharger and one or more turbochargers. During steady state operation the engine control mechanization governs these elements in a closed-loop mode by continuous adjustments that seek desired target values (“setpoints”) for particular air flow parameters in order to maintain efficient operation with low emissions. When a demand for increased torque is made, the charge air pressure must be rapidly increased (“boosted”).
Therefore, it is desirable that the air handling system of a uniflow-scavenged, OP2S engine respond to a torque request without significant delay, while maintaining control of emissions during transient operation.